Process for drilling geopressures



C. A. STUART Sept. 3, 1968 9 Sheets-Sheet 2 286 286 =86 Bod Gm: l 5.5:

M 5 I m A w I m S 8552a m w v w I W. B. L E3 55:: 22: I a m 5.2 255 55s: 253:2? r 5.52%. 5: 55s: :22: 55:2 2 is: L m. 2 .52: 5 & uu maw 5o: ammo I :c

HERE 55 3 559:6 H555: 3 E

HIS ATTORNEY Sept. 3, 1968 c STUART 3,399,723

PROCESS FOR DRILLING GEOPRESSURES Filed Oct. 10, 1966 9 Sheets-Sheet 3 SURFACE BOTTOM HOLE PRESSURE P Sl HYDROPRESSURES w w 8 w W a: u.| g a 2 E w :3 Lu 8 a: l a. :1: o a g o 32 z 3 LL] Llc INVENTOR C. A. STUART HIS ATTORNEY FIG. 3

Sept. 3, 1968 c. A. STUART 3,399,723

PROCESS FOR DRILLING GEOPRESSURES Filed Oct. 10, 1966 9 Sheets-Sheet 4 EQUIVALENT MUD WEIGHT-PPS 0.00 15.550 09 1060 0.35 Is es 20.00 .606 .693 .199 .see .953 1.039 |.|2s

GEOSTATIG QVERBURDEN GRADIENT PSI/FT.

FIG. 4

.. HYDROPRESSURE 10500- (0.465) SHALE -4000 DEPTH-FEET -e000 HYDROPRESSURE mm (0.465) SAND HYDROPRESSURES 0002 s 44,000 RE SURES HYDROPRESSURE GEOPRESSURE PQROSITY CURVE 0F SAND AND SHALE 0 v i INVENTORI 70 60 50 40 30 20 I0 0 STUART POROSITY X:

HIS ATTORNEY Sept. 3, 1968 c. A. STUART PROCESS FOR DRILLING GEOPRESSURES 9 Sheets-Sheet 8 Filed Oct. 10, 1966 CE -FES o o o o o o 0 m M m w w m w m 8 I a. 5.. H. mm W. H I M .l W 0 0 1M fi wwwfiowu A E 8:38:23: U B 0 R 5 28.; Essa s a w N 5 Es :3 m .9 mm m u L A o 5:; 2:. a I P P w H w 5 $3 0. lmd m w .l MKLIIIIIAV H U l U I. m S 0 "A F. m mm .:m w 5:-.5: m Lu l A 0 0 0 0 RU ln-L l. m m .0. w m m m mu 0 2 4 w w w n M w m H% 0 F b! 0 6 "4 v w R 01 w q W W o 4 u s m w m o o 0 SI 9 0 0 0 YT 6.. no l W. 4 w OLAG .l 3 G 0 U F 2 M o wL C. A. STUART 15% KM FIG. 9C

HIS ATTORNEY Sept. 3, 1968 c. A. STUART P OCESS FOR DRILLING GEOPRESSURES Filed 001.. 10, 1966 9 Sheets-Sheet 9 Em: LEE 0 o 0 0 m m m m w. 7 8 9 w H w n F 0 1:33:22: v o @3353 M M R a 5225 5:; :3 m A N L m1 m R m E n mm o P v A F l mu o 0 0 v M NW we] 8 0 0 o 0 o 0 0 5 E 0 0 0 0 o 9 4" V II I J1 .W J0 00 0 m E B mm m 0| W3 G m n .I F mm H 5 W 0m ool w C. A. STUART a Qwmd addq/ FIG. |oc

H IS ATITORNEY 3,399,723 PROCESS FOR DRILLING GEOPRESSURES Charles A. Stuart, Metairie, La., assignor to Shell Oil Company, New York, N.Y., a corporation of Delaware Continuation-in-part of application Ser. No. 357,485,

Apr. 6, 1964. This application Oct. 10, 1966, Ser.

25 Claims. (Cl. 166-4) ABSTRACT OF THE DISCLOSURE A method for drilling a borehole in a hydropressuregeopressure environment wherein a light weight mud is used to drill the hydropressure section of the borehole. The light weight mud is used until the target depth is reached or a kick is encountered and penetration of the geopressure section confirmed. A kick is defined as a flow into the borehole caused by the effective hydrostatic pressure of the mud column being less than the pore pressure of the formation. The kick is controlled and the borehole may be drilled deeper or protective casing may be set.

The invention pertains to the drilling of Wells in search of minerals and/or energy and more particularly to the drilling of those well which encounter or penetrate geopressure formations. This application is a continuation-inpart of copending application Ser. No. 357,485, filed Apr. 6, 1964, now abandoned.

The problems encountered in drilling a well that is drilled near and/or penetrates geopressures can best be understood by 1) defining the terms used in describing the physical and chemical characteristic of the sedimentary deposits and their geological pattern and (2) discussing the mechanics of drilling the sedimentary deposits in relationship to their characteristics and geology.

It is well known that the sedimentary deposits, hereinafter referred to as formations, in the earths crust contain pores which are filled with fluids. These fluids are pressured, and the pressures are commonly expressed quantitatively in pounds per square inch, hereinafter abbreviated as p.s.i. The pressure at any depth in the earths crust if divided by the depth in feet is then expressed as a pressure gradient in pounds per square inch per foot, abbreviated as p.s.i./ft. Pressure gradient can be converted to a density in grams per cubic centimeter, abbreviated gms./cc., or to the more commonly used weightvolume measurement of pounds per gallon, abbreviated p.p.g. As is illustrated in the following table, the pressure gradient can vary from .433 p.s.i./ft. for fresh water to 1.86 p.s.i./ ft. for barytes. In the United States the measurement in p.p.g. is most commonly used in drilling, and the measurement in p.s.i. is most commonly used in dealing with pressures.

The fluid predominantly present in the formation pores is water containing various amounts of dissolved salts, principally sodium chloride, ranging from fresh water to saturated salt water. On a small percentage basis, these pores are found to contain hydrocarbons (oil, oil and gas, gas and condensate, and gas) and on a very rare percentage basis, some non-hydrocarbon gases. The densities of the formation waters under standard conditions are 1.00 gm./cc. (0.433 p.s.i./ft, or 8.33 p.p.g.) for fresh water, 1.02 gms./cc. (0.443 p.s.i./ft. or 8.52 p.p.g.) for an average sea water, and 1.20 gms./cc. (0.52 p.s.i./ft. or 10.0 p.p.g.) for saturated salt water. The pore pressure to a certain depth is found to be generated by and virtually equal to the hydrostatic pressure of the waters contained in the formations. It is not implied that the pressure is generated directly by the overlying waters as the pressure might be due to the weighted average of waters that are contained 'ntted States Patent 3,399,723 Patented Sept. 3, 1968 TABLE I Gradient, Density, Fluid weight p.s.i./foot grns./cc.

P.p.g. P.c.f

Fresh water- 0. 433 1. 00 8. 33 62. 36 Sea Water. 0. 443 1.02 8. 52 63. 73 68,000 mg./l. C 0. 465 l. 07 8. 94 66. 87 0. 1.15 9. 62 71. 96 Saturated water 0. 52 1. 20 10. 00 74. 80 0. 55 1.27 10. 58 79. 14 0.60 1. 38 11. 54 86.32 0. 1. 50 12. 50 93. 50 0. 1. 62 13. 46 100. 68 0. 1. 73 14. 42 107. 86 0.80 1. 15. 38 115. 04 0. 85 1. 96 16. 35 122. 30 0. 90 2. 08 17. 31 129. 48 0. 2. 19 18. 27 136. 66 1. 00 2. 31 19.23 143. 84 Shale mineral 1. 10 2. 54 21. 15 158. 20 Shale and sand grain 1. 147 2. 65 22.07 165.10 1. 20 2. 77 23.08 172. 65 1. 30 3.00 25.00 187. 00 Barytes 1. 86 4. 30 35. 77 267. 56

Norms:

1. Mud weight in p.p.g. (pounds per gallon) X0.052=gradlent, p.s.i./it.

2. Mud weight in p.p.g. (pounds per gallon) 0.12=density, gms./cc.

3. Mud weight in p.p.g. (pounds per gallon) X7.48=pounds/eubic foot:

in a porous formation stratum that outcrops miles away. These pressures are commonly called normal pressures by the petroleum industry, but will be hereinafter referred to as hydropressure formations, or simply hydropressures. The pore pressures of the hydropressure formations at a depth may yield a pressure gradient ranging from 0.433 p.s.i./ft. up to 0.52 p.s.i./ft. Some hydropressure formations have pore pressures that yield a pressure gradient slightly in excess of 0.52 p.s.i./ ft. due to structural uplift and/ or the influence of a column of hydrocarbons. Generally, the pressure gradient of hydropressures in the cjoastal areas of the Gulf of Mexico, reflecting the weighted average of the formation waters, averages 0.465 p.s.i./ft. (8.94 p.p.g.). Thus, at 10,000 feet, the pore pressure of an average Gulf Coast hydropressure forma tion will be 4650 p.s.i. Expressing this in another way, the formation-fluid pressure at 10,000 feet in a well would be equal to the pressure of a column of salt Water having a pressure gradient 0.465 p.s.i./ft. of the same height.

The weight of the earths crust, both the solids and fluids, exerts pressures with depth commonly known as geostatic pressures, The geostatic pressure in p.s.i. at a particular depth can be divided by the depth in feet to obtain the geostatic gradient in p.s.i./ft. Apparently, the pressure in the pores of the hydropressure formations is not affected by the geostatic pressure.

In some parts of the world, particularly the coastal areas of the Gulf of Mexico, both onshore and offshore, and in other similar geological provinces of the United States, formation-fluid pressures in pores are encountered with gradients that exceed the hydropressure gradient and appear to be related to and generated by the geostatic pressures. In the Gulf Coast provinces, data are mostly available onshore and offshore of the States of Louisiana and Texas. The formations having these high pore pressures are commonly referred to as abnormally high-pressured formations in the petroleum industry, but are defined in this application as geopressure formations, or simply geopressures. The pore pressures in the geopressure formations have been observed to yield pressure gradients up to about the same magnitude as the geostatic gradient of 1.0 p.s.i./ ft. This is in excess of twice the hydropressure gradient. A pressure gradient twice the hydropressure gradient would be 0.93 p.s.i./ft. and pore fluids under this pressure at 10,000 feet would have a pressure of 9300 p.s.i. All formations were deposited at the surface at some geological time in the past and those now at depth were subsequently buried. As all geopressures once existed as hydropressures at a shallower depth and as they are generated by geostatic pressures that in turn are generated by 3 the overlaying waters and solids, geopressures may be considered to be generated in part by all the overlaying Waters and a portion of the solids. The pressure gradient of the pore fluids in the hydropressures and geopressures will sometimes hereinafter be referred to, broadly, as the formation pressure gradient.

It is also well known that the procedure of drilling a well involves the following equipment and procedure. The formations are drilled by the use of a rock bit that is at the bottom end of a drill stem. The drill stem is comprised of drill collars just above the bit and then drill pipe to the surface where a Kelly joint is connected. The drill collars are normally used to provide weight on the bit. The Kelly joint functions in conjunction with the surface rotary equipment such that the entire drill stem can be rotated. Fluid is pumped into the drill stem through a swivel connected to the Kelly. Thus, the fluid is normally circulated down through the drill stem, through the rock bit, and returns up to the surface in the annulus between the borehole and the drill stem. The fluids used in drilling are either in a gaseous or liquid state; the gas may be either a hydrocarbon gas or air, while the liquid is either water-base or oil-base, generally with solids dissolved or in a colloidal suspension, and is commonly called mud. Gas is used to drill hard-rock formations, while mud is used to drill both hard-rock and soft-rock formations. The mud column under static conditions yields hydrostatic pressures with depth. During mud circulation, the mud pressure in the hole may be higher than the hydrostatic pressures. Also, while running the drill pipe into the hole, the bottomhole pressure may be higher than the hydrostatic head due to the plunging effect, and, when pulling out of the hole, with the resultant swabbing effect, may be less than the hydrostatic pressure. Thus, in this application, the term effective mudhead pressure or effective pressure gradient will occasionally be used to describe the resulting pressures and pressure gradients of the aforementioned events.

The above pressure changes, i.e., negative for swabbing effect and positive for plunging effect, are a function of mud properties, length of drill pipe being moved, hydraulic diameter (diameter of the hole less the diameter of the drill stem), configuration of the drill stern, and the speed of the pipe being moved. The movement of drill stem can be considered as having a piston effect.

It has long been recognized that a well that is drilled near and/or that penetrates the geopressure formations involves high costs and appreciable risks to equipment and personnel.

The presently used procedures in the petroleum industry in drilling a hydropressure-geopressure geological column is described with particular reference being made to the casing and mud-weight programs. The procedures are observed to be quite variable but none have been developed for the hydropressure-geopressure geological formations. Generally, conductor casing is installed from 30-120 feet onshore and 400 to 1000 feet offshore. In offshore wells, a caisson (large diameter casing) is driven to refusal before drilling conductor hole and cementing conductor casing therein. Surface hole is then drilled and surface casing cemented to depths ranging from 1000 to 6000 feet with the depth being selected at random. The mud weights used in drilling hydropressures below surface casing follow all sorts of patterns. Frequently, the same mud weight will be used that was used in the closest well. Sometimes, a mud weight will be used similar to the following specific example: 6000 to 8000 feet, 11- ppg mud; 8000 to 10,000 feet, 12 p.p.g.; 10,000 to 12,000 feet, 13-p.p.g. mud; and 12,000 to 14,000 feet, l4-p.p.g. mud. In many cases, a mud weight ranging from 12 to 14 p.p.g. will be used in a long interval of hydropressures before geopressures are reached. This results in excessive overbalance and drastically retarted drilling rates. On many occasions, the mud weight will be increased until lost circulation occurs, incurring additional a 4. time and expense and sometimes stuck drill stem. Occasionally, particularly in deep wells, the drill stem will become stuck due to differential pressure. On many occasions, protective casing is installed at a depth in the hydropressures, entailing additional time and expense. Hydraulic and mechanical efiiciency of the drill stem and annulus is less in the smaller diameter hole and protective casing, than in the larger diameter hole in which the protective casing is installed, and this adds further to the reduction in penetration rates. Furthermore, tendency for differential-pressure stuck drill stem is increased in drilling hydropressures below protective pipe as the ratio of drill stem to hole diameter is larger and tendency to drill hole with higher slant and curvature is greater.

The presently used procedures for drilling geopressures will be discussed first for the case where the hydropressured formations were drilled as outlined and no casing installed. The well can be drilled into the geopressures until a depth is reached where the formation pressure gradient exceeds the mud-weight gradient, but then a kick occurs. If lost circulation is induced by increasing the mud weight to control the kick, the kick can only be brought under control by letting the hole bridge which generally results in sticking the drill stem and junking the hole.

This has led to the common practice of installing protective casing above the geopressures. A well With protective pipe installed above the geopressures section frequently will not hold a mud weight higher than if the protective pipe had not been set. In any event, if the mud weight in use is at about the formation-fracture gradient, and a geopressure kick is encountered requiring a higher mud weight, the hole will frequently be lost. The advantage of the protective casing is that if a mud weight in use is less than the formation-fracture gradient and a back pressure is required, the extra pressure gradient will be less at the shoe of protective casing than at the shallower surface casing shoe. Secondly, the hole can usually be saved to the bottom of the casing.

The aforementioned discussion is not to be construed that successful geopressure wells have not been drilled, but the successful wells are generally on a prospect where several wells have been drilled and have been cased just above the depth that a previous well was junked and abandoned and located in the same fault block. In other words, successful geopressure wells are rare and occur when the mud and casing program happens by luck to fit the hydropressure-geopressure formation.

In viewing the procedure where protective casing is installed in hyd'ropressures, a protective liner is then required at the depth selected on the above basis. There is still risk that the well will not reach its target objective since a considerable number of reduced diameter casing or liners will be required as the well penetrates deeper formations in the geopressures. Thus, the possibility exists that the casing will become so small that it would be impossible to drill within the casing or liner string.

In the event the protective casing is installed just above the depth that a previous well was junked and abandoned, the well will still be lost if a liner is not installed to eliminate the high differential pressures near the shoe or before a kick is encountered that exceeds the formationfracture gradient at the top of the geopressure section. Again premature setting of liners below protective casing may result in the well not reaching the target objective and incur unnecessary expense.

In view of the above problems, it is the principal object of this invention to provide new techniques to drill the hydropressure section and new techniques to drill the geopressure section of wells that are drilled near and/or that penetrate the geopressure formations. Simply stated, it is the objective of this invention to drill the safest, fastest, most economical well to the target objective, with no limit being placed upon depth of a target objective save the limit of mechanical and rig equipment, by fitting the mud-weight and casing program to the hydropressuregeopressure formations being drilled.

This invention solves the above problems by providing a method for drilling wells near and/or that penetrate geopressure formations, comprising the following steps:

(1) Install surface casing with most economical design at the shallowest depth based on the compaction of the formations such that the mud weight that can be carried below and the chances of bringing kicks under control are optimum.

(2) Determine the pressure gradients in the hydropressure section below the surface casing to permit the use of lowest possible drilling fluid weight to provide the fastest penetration of the hydropressure section.

(3) Determine the depth that the top of geopressures occur and preferably determine the pressure gradient of the geopressures. It is paramount to this invention that protective casing be installed into geopressures and desirable that it be installed at the maximum and safest depth into the geopressures that can be penetrated without exceeding the fracture gradient of the hydropressure section. This permits using the maximum possible mud weights below protective casing without lost circulation.

(4) After installation of the protective casing, determine the formation pressure gradient of the geopressure formations to permit the use of the lowest fluid weight to provide the maximum penetration rate of the geopressure formations.

(5) Determine that depth below protective casing at which the differential-pressure sticking conditions are apt to cause the drill stem to stick, and installing a protective liner before such sticking conditions are reached.

The above objects and advantages of this invention will be more readily understood from the following detailed description of a preferred embodiment when taken in conjunction with attached drawings in which:

FIGURE 1 is a diagram of pressure versus depth illustrating the formation and geostatic pressure-gradient distribution and nomenclature;

FIGURE 2 is a diagram of formation-fracture pressure gradients superimposed on the formation and geostatic pressure gradients;

FIGURE 3 is a diagram of differential pressure of the hydrostatic mud pressures less the formation pressures in the hydropressures and geopressures;

FIGURE 4 is a diagram of the geostatic overburden gradient with depth.

FIGURE 5 is a diagram of the porosity of sand and shale formations.

FIGURES 6-10 are the drilling records of wells using the techniques of this invention and wells that did not use the techniques of this invention.

FIGURE 1 shows that formation-pressure gradient of the pore fluids of geopressure formations increases from the hydropressure pressure gradient to higher values with depth. The actual pressures and pressure gradients in this figure are merely representative and are not fixed values for all formations. As shown, the top of the geopressures is assumed to be at 10,000 feet. Proceeding deeper from this depth, it is seen that the pore-pressure gradient increases from 0.465 to 0.75 p.s.i./ft., an arbitrary figure, is defined herein as the mutation zone. The interval of the mutation zone may range from about 200 feet to about 1200 feet. Proceeding deeper it will be noted that the pressure gradient may be constant for a certain interval, then increase again. Proceeding farther with depth, the geopressure pressure gradient is indicated to continue to increase and has been observed to approach the geostatic pressure gradient which, as illustrated, is about 1.0 p.s.i./ ft. at 20,000 feet.

As shown in FIGURE 5, pore volume of the formations may be about 70 percent for muds and clays and around 40 percent for the sand and gravel deposits at the surface of the earths crust. With depth of burial in the hydropressure section, the porosity is reduced due to compaction and the rate of porosity reduction is quite rapid to about 2,000 feet after which the porosity reduction continues, but at a slower rate throughout the entire hydropressure section. In hydropressures, this compaction is caused by the weight of all the overlying solids acting on the grains of the sand and shale formations, as the overlying hydrostatic head of waters is supported and compensated by the hydropressure pore pressure.

As shown in FIGURE 4, the compaction force is the geostatic pressure less the hydropressure pore pressure. In the geopressures, however, the effective compaction is less than that in the hydropressures because the weight of the overlying solids is partly supported by the geopressure pore pressure. When the pore pressure approaches the geostatic pressure, all the overlying weight (geostatic pressure) acting on the grains of sand and shale is almost entirely supported by the pore pressure. Therefore, the grain-to-grain compaction force becomes less as the pore pressure increases. Consequently, there is a change in the porosity-reduction trend and porosities increase as the geopressures are entered and penetrated.

A sand or shale with a higher porosity, all other features being equal, will have a lower sediment (bulk) density, meaning weight per unit volume of the rock (mineral grains and pores), than a sand or shale with lower porosity. The porosity trends and densities of the minerals and contained waters of the lithological column yield an increasing geostatic gradient with depth. Because of the porosity trend in geopressures, the geostatic-increase trend in hydropressures is reduced in the geopressures and a geostatic reversal would be physically possible but has not been observed. As shown in the example in FIG- URE 4, the geostatic gradient is 0.85 p.s.i./ft. at 2,000 feet, 0.95 p.s.i./ft. at 12,000 feet, and reaches an estimated 1.0 p.s.i./ft. at 20,000 feet. Formation-pressure gradients of 0.94 p.s.i./ft. have been observed, and, with deeper drilling, slightly higher pressure gradients are anticipated. Thus, it is seen that the pore-pressure gradient of geopressures reaches up to 94 percent of the geostatic gradient and may be found to be slightly higher in the future.

The formation that exist in a typical hydropressuregeopressure geological column on the Gulf Coast are normally shale, sandy shales, shaly sands, sands, and sometimes limestone and calcareous formations. In drilling a well, it has been a long-established custom to contain the fluid pore pressures of these formations with the use of a mud column that yields a hydrostatic pressure that is suflicient to overbalance or exceed the formation pore pressure. The difference between the hydrostatic pressure of the mud column and the pore pressure is defined as differential pressure or overbalance. If the effective hydrostatic pressure of the mud column is less than the pore pressure, defined herein as being underbalanced, the following events will occur. If the formation is permeable and contains salt water, a salt water flow will result. If it contains oil, an oil flow occurs, and if gas, a gas flow results. A flow may be a combination of one or more of these fluids. If the formation has low permeability, gas, salt water, and/or oil may enter the well bore but may not be visually observed at the surface by surface pressure, velocity and volume of mud returns, or other indications that fluids are entering the Well until the mud is circulated to the surface with a detectable gas, salt water, and/ or oil cut. Sometimes the permeability of a formation, generally shale, is so low that the formations heave. This heaving shale condition may also be called caving or sloughing shale and has a tendency to stick the drill stem. These shales, and sheath, to be defined hereinafter, will not heave, however, until a certain amount of underbalance exists. All of the above events that occur when the effective hydrostatic pressure of the mud is insufficient to contain the pore pressure of the formations is defined herein with the broad term of kick. A kick may be observed to be a combination of more than one of the above events. For example, a kick may be a combination of heaving shale and gas-cut mud. As the formation pressure gradients increase from the hydropressure gradient at the top of the geopressures to higher gradients approaching geostatic with depth, it is easily seen that the depth below the top of the geopressures where a kick will be encountered is a function of the mud weight. The higher the mud weight, the deeper the well will be before a kick is encountered.

Soft-rock formations are defined as those geological formations in which the shales heave with underbalance. Soft-rock formations generally contain frequent permeable layers that will yield oil-gas-saltwater-flow type kicks as well as the oil-gas-saltwater-cut type kicks and heavingshale type kicks. Soft-rock formations are generally contained in the Cenozoic era and require mud to drill. Hard rock formations are defined as those geological formations in which shales will not heave with underbalance, although sometimes long intervals are found in hard-rock formations that yield only the cut type kick. Appreciable hard-rock hydropressure intervals can be drilled with gas.

When a kick is encountered, an additional pressure can be added immediately at the surface by closing the blowout preventer, hereinafter abbreviated BOP, and then forcing the mud returns through a choke manifold. The choke manifold can be adjusted to impose any amount of back pressure. A back pressure can be selected, which when added to the effective head of mud can balance or slightly overbalance the pressure of the kick formation, thereby stopping entry of fiuids into the well bore. The mud weight can then be increased to that amount which will yield a hydrostatic head to slightly overbalance (killing) the kick, after which the BOP equipment can be opened. Most of the time a kick yields fluids into the well bore at such a slow rate that the kick can be killed by increasing the mud weight without closing the BOP. When the BOP is closed, imposing a back pressure, this will yield an additional mud pressure gradient at the bottom of the hole and impose progressive higher pressure gradients with progressive shallower depths. For example, a 520-p.s.i. back pressure will impose an extra 1.0-p.p.g. pressure gradient equivalent at 10,000 feet, but will impose an extra 3.3-p.p.g. pressure gradient at 3000 feet.

In hydro-pressures, a salt water flow, unless lightened with gases, will merely fill the hole to the surface as the hydrostatic head of salt water then becomes equal to the pore pressure and contains it. Under drilling conditions, the swab effect could induce a swab kick in hydropressures. A hydropressure gas or oil flow would produce fluid to the surface with a column of fluid yielding a pressure less than the pore pressure, and, thus produce surface pressure. In other words, oil and/or gas from a hydropressure formation will produce surface pressure and energy.

In geopressures, the pore pressure exceeds the pressure of a column of salt water in the well. For example, if a geopressure salt water sand at 10,000 feet has a pressure of 9300 psi, pressure gradient of 0.93 p.s.i./ft., the formation would yield a shut-in surface pressure of 4650 p.s.i.

To further explain the techniques of this invention, reference is made to FIGURES 2 and 3 to discuss the formation-fracture pressure gradient (lost circulation) and differential-pressure gradient (overbalance, or excess of the effective mud gradient over the pore-pressure gradient). The difficulty of drilling a well in the hydropressure geopressure geological environment stems from the fact that an adverse relationship exists between and among these gradients and the formation pore-pressure gradient.

The formations down to about 2,000 feet, if uncased, generally will hold an effective column of mud ranging from about 9.0 to 13.5 p.p.g. (0.467 to 0.703 p.s.i./-ft.) before a formation or formations are hydraulically fractured and lost circulation occurs. The interval from about 2,000 feet to the top of the geopressures will hold a mud weight ranging from 11.0 to 16 p.p.g. In viewing an interval of hydropressure formations, most individual formations will hold about a 16.0-p.p.g. mud, but a few will be weaker. These weak zones are commonly called thief zones and are believed to be associated with faulting. Thief zones exist throughout the geological column. The open hole (uncased) of a geological interval is just as strong as its weakest formation. In other words, the hydropressure section of some wells, with surface casing installed at 2,000 feet, may hold only an effective 11.0- p.p.g. mud before a formation is fractured and circulation is lost. Continued pumping of mud weighing more than 11.0 p.p.g. into the drill stem results in the mud being pumped into the fracture or fractures.

A thief zone as well as any zone has a fixed, effective hydraulic fracture pressure. Once fractured, however, the formation will take fluid at a lesser pressure. The latter pressure divided by the depth of occurrence will hereinafter be referred to as the formation pump-in gradient. There appears to be no way to increase the fracture gradient of a formation, and it is not possible to use mud weights higher than the formation fracture gradient unless the formation has been cased off. For example, if a well is being drilled in a formation having a 14.5-p.p.g. pressure gradient, using a 15.0-p.p.g. mud, and a formation is fractured that has a formation pump-in gradient of 14.0 p.p.g., a kick is immediately triggered that may not be possible to control before the open hole is junked, as hereinafter defined.

The average hydropressure section will hold about 12.5- to 13.5-p.p.g. mud before lost circulation occurs. On the other hand, some hydropressure wells will hold about 16.0-p.p.g. mud before lost circulation occurs. In the geopressures, however, the formation-fracture gradient abruptly increases, and geopressure sections, where the lowest formation-pressure gradient is at or above 14.5- p.p.g., have been observed to hold the equivalent of 21.0- p.p.g. mud. In the mutation zone, the formation-fracture gradient increases as the formation pressure-gradient increases.

If casing is installed and is perforated opposite a hydropressure formation, it generally requires an effective 16.0-p.p.g. mud to fracture the isolated formation by squeezing. If the casing is opposite geopressures and perforated, a fracture pressure equivalent to an effective 25-p.p.g. mud has been observed but only infrequently, and this gradient is illustrated in the figure. Open-hole geopressure intervals, however, have not been observed to hold this high a mud weight. It is seen that if a Well, cased with surface casing, is being drilled with a l3.5-p.p.g. mud that is just at the formation-fracture (lost circulation) m-ud weight and encounters a salt water sand that is under a pore pressure yielding a pressure gradient of 14.5 p.p.g., a salt water flow will occur. It is impossible to bring the flow under control by increasing the mud weight because any increase of mud weight merely induces lost circulation. It the kick is not kept under control at the surface, a blowout would occur. When the well is kept under control at the surface, the only hope of controlling the kick is letting the hole bridge, shutting off the kick formation. This bridging effect generally occurs in this example type of kick, frequently sticking the drill stem. The hole is then junked and abandoned.

The aforementioned condition of a geopressure kick is one of the factors that has led to the inclusion of the so-called Gulf Coast Escape Clause in the drilling contracts of wells in the onshore and offshore provinces of the Gulf of Mexico. The existence of salt domes and sheath surrounding the salt dome is also included with the geopressure kick in the escape clause. Sheath is defined herein as the disturbed zone between the normal sedimentary deposits and the salt dome and exhibits similar pore-pressure distribution as geopressure formations except sheath is more erratic and may show higher increases in pore-pressure gradient per depth of penetration than geopressures. The expression impenetrables is usually included in the escape clause. In other words, if a drilling contractor enters into a contract with an oil operator to drill a well to 10,000 feet and encounters salt, sheath, a kick, or sticks the drill stem, rendering deeper drilling either very expensive or impossibe before reaching 10,000 feet, he is relieved from further responsibility through the Gulf Coast Escape Clause.

Thus, it is seen that, because the pore-pressure gradients of geopressures are higher than the lost-circulation gradient of the hydropressures, only a short distance can 'be drilled into the geopressures before a depth is reached where no additional penetration can be made without the techniques hereinafter described.

The problems arising in drilling the hydropressuregeopressure formations are further complicated by the differential-pressure gradient pattern, as illustrated in FIGURE 3, as related to the hydropressure-geopressure geological column. In the hydropressure section, the differential-pressure gradient may be increased by increasing the mud weight up to the formation-fracture gradient. Assuming this is a 14.0-p.p.g. mud, the differential-pressure gradient would be approximately 14.0 less 8.9 p.p.g., or 5.1 p.p.g., and in other used terms, 0.73 p.s.i./ft. less 0.465 p.s.i./ft. or 0.265 p.s.i./ft. Using this constant mud weight, it is seen that the differential pressure increases with depth. For example, at the surface it is zero; at 5000 feet, it is 1325 p.s.i.; and at 10,000 feet it is 2650 p.s.i. Thus, in the hydropressure section, differential pressure increases with depth. In the geopressures, however, differential pressure reverses with depth. For example, in a geopressure section from 13,000 feet, Where the formation-pressure gradient is 0.6 p.s.i./ft. (11.54 p.p.g.), 17,000 feet, where the formation-pressure gradient is 0.9 p.s.i./ft. (17.3 p.p.g.), and in which the hole is filled with 18.0-p.p.g. mud (0.935 p.s.i./ft), the differential pressure would be 4360 p.s.i. at 13,000 feet and 595 p.s.i. at 17,000 feet.

Differential pressure (overbalance) induces several problems in drilling. Experience indicates that drilling, or penetration, rates are retarded by increased differential pressures. For example, the time to drill hydropressure formations is approximately doubled when a 12-p.p.g. mud is used as compared to drilling the same formations with a l-p.p.g. mud. Also when the overbalance is decreased, a point is reached where the pressures are balanced and a further reduction will result in an underbalance. In practice an imbalance is only achieved in gas drilling that in turn results in maximum penetration rates. When geopressures are penetrated an overbalanced mud, additional drilling will reduce the overbalance and increase the penetration rate.

As aforementioned, the differential pressure that a formation will sustain is a fixed physical amount, and any excess of this amount will cause lost circulation. Another drilling problem of differential pressure is the sticking of the drill stem. The tendency of the drill stem to become differentially-pressure stuck is related to force of the drill stern into the wall, net thickness of porous formations in which a filter cake of the mud has built up, the thickness of the filter cake, the coefficient of friction of the filter cake and the drill stem, duration of drill stem in a stationary position, ratio of drill-stem diameter to hole diameter, and differential pressure. In a. strict sense, no borehole of any well is perfectly vertical; consequently, there is always some slant and curved hole. The force into the wall is a function of weight of the drill stem in slant hole and amount of slant. There is also a force into the wall which is a function of the tension of the drill stem below the position of curved hole and the degree of curvature. In hydropressure hole, the highest differential pressure is at the bottom of the hole where the drill collars are located that provides the largest ratio of drill-stem to hole diameter and highest weight per unit length of drill stem. Consequently, in hydropressure wells, the greatest tendency for differentially-pressure stuck drill stem is at the bottom of the hole.

In a geopressure section, however, the highest differential pressure is at the top of the geopressure section, and, with this geopressure section, this is also at the position where maximum tension of the drill stem exists which yields the maximum force of drill pipe into the wall due to curved hole. Consequently, the tendency for differentially stuck drill stem is near the top of the geopressure section. The deeper the section is drilled into the geopressures, the greater is the tendency of sticking the drill stem, as tension is increased and differential pressure is increased.

Geopressures appear to be associated with more frequent paleontological markers than the typical hydropressure section. There appears to be a different geological depositional environment for geopressures than hydropressures. Regionally, a hydropressure-geopressure province contains many faults. Sometimes the pressures from geopressures have seeped up the fault gouge in the hydropressures. All geopressure reservoirs and some hydropressure reservoirs just above the geopressure-s are sealed reservoirs. Withdrawals of fluids will decrease the pore pressures and input of fluids will increase the pore pressures. The province also contain-s structures and most hydrocarbons are found trapped in these structures. These structures are very highly faulted generally. In the hydro pressure section, uplift will not affect the pressure gradient of a salt water sand appreciably. The pressure gradient will increase with uplift in a gas or oil sand. In geopressures, the pressure gradients of salt water, oil and gas sands increase with uplift. The increase of pressure gradient is highest for gas, then oil, followed by salt water because of lower to higher densities of the three fluids. The formation-fracture gradients are lower in a geological column that is highly faulted than one with few faults. The top of geopressures generally follow a stratigraphic level, or the level of a formation in a particular fault block. Across faults, the top of the geopressures may be found higher or lower stratigraphically, and higher or lower in measured depth. For example, across one fault in Louisiana, the top of the geopressures are some 1500 feet higher in measured depth and 10,000 feet higher in stratigraphic depth than on the other side of the fault.

In considering geological age, the top of the geopressures have been observed in the onshore-offshore province of Louisiana to occur in the Eocene, Oligocene. Miocene, and Pliocene-Pleistocene. From the standpoint of depth, the top of the geopressures have been found to occur higher than 5000 feet and deeper than 19,000 feet. These geological aspects illustrate that no two wells, except perhaps twin wells which are from the standpoint of geology one well with a very large diameter, will penetrate precisely the same hydropressure-geopressure geological column. There appears to be a difference in the pattern of the salinities of the formation waters in the geopressures as compared to the hydropressures. Depending on the amount and proximity of hydropressure-geopressure drilling done in an area and knowledge of the invention covered in this application and copending applications, data will be available to determine the general picture of the hydropressure-geopressure column of a well to be drilled but usually it will not be known precisely until the Well is drilled. This has led to problems in planning mud-weight and casing programs.

The present invention solves the problems of drilling in or near geopressures by the following novel procedure:

Following the installation of conductor casing, as described above, the surface hole is drilled with a mud weight yielding a differential pressure only to the extent that is required to overcome the swabbing effect under drilling conditions. Surface casing is installed at the depth (1) based on a study of the geostatic gradient, (2) where shallower fault zones and weak zones will be cased, (3) where any additional depth of installation would have little effect on the mud weight that the open hole would hold below, and (4) that any expected kick would not require a surface pressure that with the hydrostatic head of mud would induce lost circulation in the hydropressure section below the surface casing. Usually surface hole should be drilled with a mud weight ranging from 9.0 to 1l.0-p.p.g. mud and surface casing should be installed from 2000 to 4000 feet. Shallower installations will be sufficient if the target objective of the well is considerably above the expected top of the geopressures.

The optimum depth for installing surface casing can be obtained by referring to FIGURES 4 and 5. FIGURE 4 is the geostatic gradient of FIGURE 1 drawn to an enlarged scale to illustrate that the geostatic gradient increases at a rapid rate until about 4000 feet. Beyond 4000 feet the gradient increases at a slower and substan tially constant rate. Thus, 4000 feet is the deepest that surface casing should be set. FIGURE 5 illustrates the porosity of sand and shales. It should be noted that the porosity of shales crosses the porosity of sands at 2000 feet which is the shallowest depth that surface casing should be set.

After surface casing is installed, the hydropressure section is drilled with a mud weight yielding a differential pressure higher than the formation-pore pressure to the extent that is required to overcome swabbing under drilling conditions. As the hydropressure formation usually has a pressure gradient of 8.94 p.p.g. (0.465 p.s.i./ft.), the mud weight can generally be carried between 9.5 to 10.5 p.p.g. Where the hydropressure formations have higher pressure gradients, slightly higher mud weights will be required but the amount of overbalance should still be maintained for only the swabbing effect.

This relatively light mud weight is carried throughout the entire hydropressure section. This procedure is followed until geopressures are entered when drilling is halted. If geopressures are determined by a kick, the mud weight is increased until the kick formation is controlled, with or without the use of BOP and choke manifold as required. The geopressure kick establishes that the well has entered geopressures and the pressure of the kick formation can be determined.

It is noticed by the technique of using relatively light mud weight, there would be maximum difference between the mud weight in use and the formation-fracture gradient (lost circulation) of the hydropressure open hole. There would also be maximum difference in the pore-pressure gradient of the kick formation and the formation-fracture gradient of the hydropressure open hole. Thus, the physical conditions are optimum that the kick can be brought under control.

It should be mentioned, however, that steps should be taken to determine if the indications of a kick were a result of a geological" or mechanical condition rather than due to underbalancing a geopressure formation. For example, a swab show may occur after the drill stem has been picked up off bottom and the mud circulated to the surface. It generally requires a little extra pull to pick up the drill stem under these circumstances. Should the kick come from a fault zone in the hydropressure section, the existence of the fault can often be determined from geological data provided by electrical log, paleontological log, and lithological log correlations. Should the kick indication come from a drill show of a hydrocarbon-bearing sand or from underbalancing in a hydropressure sand, this can be determined by various surveys through the drilled interval. All the above kick indications are of short duration, that is, fluid entry does not continue until overbalanced with a weighted mud as does the geopressure kick, except the latter und-erbalanced sand.

Furthermore, any log or survey, such as self-potential, resistivity, induction, sound velocity, density, neutron, chlorine, temperature, penetration rates, rock drillability, etc., in which either porosity, pressure, differential pressure, or thermal gradient is a parameter will show a shift in the types of formations as geopressures are entered, providing additional techniques or methods of determination of geopressures. For example, resistivities from the electrical log reflect the porosities, which reflect the pore pressures; and pressures can be determined from shale resistivity as covered in a copending application of C. L. Blackburn, C. E. Hottman and R. K. Johnson, Ser. No. 226,937, filed Sept. 28, 1962.

Another method uses acoustical logs, also known as the sonic, acoustic, or other trade name or names, which are described in the copending application of C. E. Hottman, Ser. No. 144,685, filed Oct. 12, 1961.

Using the processes of this invention one can detect the top of the geopressures, the pressure gradient over the entire hydropressure geopressure column as the well is drilled and the differential pressure. From this information the optimum mud weight can be selected in addition to the proper depth to set casings and liners. One of the most important steps of process is that geopressures must be entered before installation of the first string of protective casing below the surface casing.

The character of the formations in the mutation zone will control which method of geopressure detection will provide the first indication of geopressure penetration. 1f the zone contains formations that will yield fluids or heave with small underbalance, the resulting kick will be the first indication. If the zone contains formations that will withstand a high underbalance, then a logging method will provide the first indication of entry into the geopressures.

The depth of the first kick provides a good protective casing point, however, depending on strength of the hydropressures to fracture and knowledge of the pressure-gradient increase of the mutation zone, and whenlittle risk is involved, some additional penetration into the geopressures may be warranted before installing protective casing. This can be accomplished either by drilling into a second kick using the same mud weight used to control the first kick or increasing the mud weight a specified rate per unit depth such that the overbalance is maintained at about 0.5 to 1.0 p.p.g. and the formation pump-in gradient is not reached. The procedure, in which additional drilling below the first kick is done, increases the formation-fracture gradient below the protective string and eliminates the differential pressure of the geopressure interval then behind the casing.

Furthermore, when information is available to determine the formation pressure gradient as the geopressures are being drilled, it would be possibleto increase the mud weight as the geopressures are being penetrated; maintaining the desired overbalance, or alternatively it would be possible to increase the mud weight to a specified amount just above the geopressures, and to drill'the desired interval into the geopressures. The protective casing would then be installed into the geopressures, and no kick would have been encountered.

Drilling below the protective string can be varied depending on the knowledge of the pressure gradient of the geopressures. If the pressure gradient is known or can be determined while the formations are being drilled, the mud weight can be increased such that a 0.5- to 1.0-p.p.g. mud overbalance is maintained at all times. If the pressure gradient is unknown, three general procedures can be followed. The first involves drilling ahead with the'same weight until kicks are encountered and controlled. The second involves increasing the mud weight at a low fixed rate, say 0.2 p.p.g./ feet which would bypass'kicks where the pressure gradient also increases at a low rate. The third procedure entails increasing the mud weight at a high rate, say 1.0 p.p.g./100 feet, which would bypass kicks of most geopressure formations. After reaching a 16.0 p.p.g.-mud in all the above procedures, the mud can then be increased at a low rate or drilled ahead at the same mud weight, increasing as kicks are encountered, until a 17.5-p.p.g. mud is reached. Kicks encountered with mud weights above 16.0 p.p.g. are generally mild. The average well, with casing installed into the geopressures, will hold about 17.5 p.p.g. mud. Some wells, however, hold a higher mud weight while other wells hold a lower mud weight.

At the depth where 16- to 17.5-p.p.g. mud is required, the hole conditions should be reviewed for installation of the protective liner. Depending on the formation-fracture gradient and the tendency of the drill stem to stick due to differential pressure, and the geopressure formation-pressure gradient, usually some 1000 to 6000 feet can be drilled before protective liner is required.

The technique involved here, however, is that there is a depth that protective liner is to be installed and that it can be determined as the well is drilled.

Following the installation of the protective liner, if it has been installed opposite a formation-pressure gradient equivalent to at least 17.0 p.p.g., there apparently is little limitation to the target objective except mechanical limitations of the drilling equipment.

From the above description of the method of this invention, it is seen that the portion of the well that penetrates hydropressures is drilled using a very light weight mud and normal drilling techniques. This results in a greatly increased rate of drilling thus reducing the overall cost of the Well. This technique provides a means to drill into the geopressures with optimum safety and confirms that the well has actually penetrated the geopressure formations. This invention provides a depth to install the first protective casing that eliminates the possibility of hydraulically fracturing the weak hydropressure formations and thus minimizes or eliminates lost circulation and the attending problems thereto.

Other important features of this invention are that it provides a procedure to drill the geopressure formations at first penetration rates and provides a depth for installation of the protective liner and, most important of all, provides a means to reach deep target objectives.

The advantages of drilling wells following the method of this invention are clearly illustrated from the following data:

For example, in the West Cameron area of offshore Louisiana, one geopressure well was drilled to a depth of 12,315 feet in 34 days at a total cost of 415,000 dollars. A well drilled in geologically similar formations by another party required 132 days and cost 1,400,000 dollars to reach the same depth.

Similarly, in an offshore Louisiana field, using the principles of this invention, a total of 9 slant wells were drilled at a total cost of 1,400,000 dollars or an average cost of 155,000 dollars per well and required 19 days each to reach the target depth that is just above geopressures.

-A second partys 9-well slant program which was carried out using the prior techniques for drilling such hydropressure formations cost $3,100,000 or an average of $342,000 per well and required 43 days to reach the same target depth.

A three-year, 58-well program in offshore Louisiana on eleven prospects or fields was drilled, using the techniques of this invention, at a cost of $18,000,000 and all wells reached their target objective. An average trend of costs with depth was established by these wells. All the wells drilled by this operator and other operators in the same eleven prospects or fields, without the techniques herein set forth, established a cost trend of four times the above 58-well program and most were junked and abandoned above the target objective. From these figures, it is easy to see that the method of this invention reduces the time required to drill wells near and into geopressures by at least /4 resulting in a corresponding reduction in the total cost of the wells.

Further examples of both the use and failure to use the above-described methods are shown in FIGURES 6-10. FIGURES 6A, B, and C illustrate the failure to use the above method in a well drilled offshore in Louisiana to a total depth of 13,719 feet. Shown in FIGURE 6B is the drilling time versus depth for this particular well, the total time being 114 days and the total cost $1,060,000. It should be noted from FIGURE 6A that the mud weight was gradually increased from 8200 feet on until the total depth was reached. It is also shown in FIGURE 6A that the casing or protective liner was installed at approximately 12,112 feet, while the well was still in the hydropressure section. The mud weight was then increased above 16.0 p.p.g. at 13,197 and at 13,373 feet, which is higher than the fracture gradient, and lost circulation problems occurred. Circulation was regained only after reducing the mud weight. The well entered the geopressure formations at 13,000 feet but did not kick until 13,719 feet as a result of the high mud weight being used. The entry into geopressures at 13,000 feet is confirmed by the plot of shale resistivities in FIGURE 6C. The well of FIGURES 6A, B and C illustrates the very slow penetration rates that are obtained with a heavy mud weight and a large overbalance.

FIGURES 7A, B and C illustrate a well drilled in the same geological province for a total depth of 15,600 feet without penetrating geopressures. This well cost $1,465,- 000 to drill and required 148 days. At 15,200 feet lost circulation problems occurred, and when the mud weight was lowered, no additional lost-circulation difficulties were experienced. The plot of shale resistivities in FIG- URE 7C shows that geopressures were not penetrated.

FIGURES 8A, B and C illustrate a well in the same geological area that was drilled to a total depth of 12,295 feet and did not penetrate geopressures. The Well was drilled with a relatively low mud weight of 9.9 p.p.g. The well shown in FIGURE 8 cost $250,000 and required 21 days to drill to its total depth. Since no kick was encountered even with the low mud weight, it is obvious no geopressures were entered. This is confirmed by the plot of shale resistivities in FIGURE 8C.

From a comparison of the times required for all of the wells to reach approximately a 12,000-foot depth, it is seen that the well of FIGURE 6 required approximatey 65 days, the well of FIGURE 7 required 40 days, while the well of FIGURE 8 required only 20 days. Since all of the wells were drilled in the same geological area and non penetrated geopressures before 12,000 feet, the figures clearly illustrate the cost in time and money of the steps taken in anticipation of penetrating geopressures.

FIGURES 9A, B and C illustrate a rank Wildcat Well drilled to a total depth of 15,082 feet at a cost of $23 5,000 that required 20 days to drill. This well was drilled with a low mud weight of 10.6 p.p.g. at total depth until a kick indicated that geopressures had been entered. The kick, gas and salt water cut was brought under control by increasing the mud weight to 11.8 p.p.g. While bringing the kick under control by increasing the mud weight, observation and calculations indicated the geopressure formation was in balance with an effective 11.4-p.p.g. mud, establishing the pressure gradient for the formation. An electrical log was run and a plot of shale resistivities confirmed that geopressures had been entered. Note that the well of FIGURE 7 took 7.4 times as long and cost 6.2 times as much as the well in FIGURE 9.

The well shown in FIGURE 9 ilustrates how geopressures can be penetrated using the kick method of this invention and the geopressures easily controlled. As shown, a 11.8-p.p.g. mud controlled the kick, and this is substantially less than the formation-fracture gradient of the hydropressure section. As shown in FIGURES 6 and 7 these hydropressure formations will hold at least a 14- p.p.g. mud weight.

Referring now to FIGURES 10A, B and C, there is shown a wildcat well that was drilled to a total depth of 12,420 feet at a cost of $740,000 that required 67 days to drill. It should be noted that this well, although not as deep as those shown in FIGURES 6 and 7, penetrated approximately 4590 feet of geopressures. Geopressures were detected when a salt water flow was encountered at 7950 feet (top is at 7830 feet) using a 10.4-p.p.g. mud that required ll.6-.p.p.g. mud to quiet. Geopressure formation pressure gradient at this depth was 10.8 p.p.g. After the well was quieted, the protective casing was installed at this point and the well was deepened. At 10,711 feet the mud weight had been increased to 17.5 p.p.g. to control the various kicks encountered as the increasing gcopressures were encountered. This mud weight yielded a pressure differential of 2695 psi. on sands below the protective casing and pressure-differential gradient of 6.7 p.p.g. (17.5 less 10.8) on the formations. Calculations indicated that the drill string would stick due to differential pressure if additional drilling; was undertaken, and a 7-inch liner was installed. The well was finally deepened to a total depth and encountered the top of the salt dome formation at approximately 12,420 feet. At this point, a mud weight of 18.5 pounds was being used. The maximum pressure differential in the open hole below the liner was only 1.3 p.p.g. (18.5 less 17.2 p.p.g.), and

neither differential pressure sticking nor formation fracturing (lost circulation) occurred.

While the well shown in FIGURE 10 did require more time to drill, it is to be noted that the penetration rate even for the deep geopressure section was substantially the same as that for the hydropressure sections. Also, the time required to drill the well of FIGURE 10 included time lost during two tropical storms. This drilling performance is in marked contrast to the penetration rates for the wells shown in FIGURES 6 and 7 where the penetration rate continually decreases as the well is deepened even though the wells did not penetrate the geopressure formations. These comparisons illustrate that the penetration rate will be greater when the overbalance is kept as low as possible. The comparisons also illustrate that it is possible to control the first geopressure kick if it is encountered using a mud weight below 11 p.p.g. They also illustrate that no mud program based on preselected depths (unless the formation pressure gradient is known before or while the well is being drilled) will prove successful as geopressure formations can occur at 7830 in FIGURE 10 and not be reached until 15,600 feet in FIGURE 7.

I claim as my invention:

1. A process for drilling a borehole in a hydropressuregeopressure environment, said process comprising:

drilling the surface portion of the borehole and setting surface casing; continuing to drill the borehole using a mud weight having a 0.5 to 1.6 p.p.g. overbalance in the hydropressures until the first geopressure kick is observed;

halting the drilling and controlling the kick by increasing the weight of the mud;

confirming that the kick was a result of the borehole penetrating the geopressure zone;

installing protective easing into the geopressures and continuing drilling while maintaining the mud weight with an overbalance of 0.5 to 1.0 p.p.g. while drilling the geopressure section; continuing to drill for 1000 to 6000 feet and then setting protective liner to eliminate differential-pressure stock drill stem and formation fracturing; and

continuing to drill using mud weight up to the geostatic gradient.

2. The process of claim 1 wherein confirmation that the borehole has penetrated the geopressure zone is obtained by logging the borehole using a logging technique that has density or porosity as one of its parameters.

3. The process Of claim 1 wherein the portion of the drillin after the setting of surface casing is conducted using a mud weight of between 9.5 and 11 p.p.g.

4. The process of claim 1 wherein the portion of the drilling after the setting of protective casing is conducted using a mud weight that is increased at a predetermined rate until a 16-p.p. g. mud weight is reached.

5. The process of claim 4 wherein the predetermined rate is one p.p.g. per feet.

6. The process of claim 1 wherein the mud weight used is higher than the formation pressure gradient only as required to overcome swabbing.

7. The process for claim 1 wherein the depth to install protective liner is based on calculations of differential-pressure sticking conditions.

8. A process for drilling a borehole in a hydropressuregeopressure environment, said process comprising:

drilling the surface portion of the borehole and setting surface casing; continuing to drill the borehole using a mud weight having a 0.5 to 1.5 p.p.g. overbalance in the hydropressures until the first geopressure kick is observed;

halting the drilling and controlling the kick by increasing the weight of the mud;

confirming that the kick was a result of the borehole penetrating the geopressure zone; and

completing the borehole.

9. The process of claim 8 wherein the hydropressure section is drilled using a mud weight that is underbalanced.

10. The process of claim 8 wherein the trend of formation porosity is measured using a logging technique that responds to porosity and a shift in the measured porosity is used to confirm that the borehole has penetrated the geopressure section. I

11. The process of claim 8 wherein a portion of the geopressure section is drilled without protective casing until the mud weight equals the formation fracture gradicut.

12. The process of claim 11 wherein the depth of penetration of the geopressure section is confirmed and at least a protective casing is installed.

13. A process for drilling a borehole in a hydropressuregeopressure environment, said process comprising:

drilling the surface portion of the borehole and setting surface casing,

continuing to drill the borehole using a mud weight having a 0.5 to 1.5 p.p.g. overbalance in the hydropressures until the desired depth of the borehole or the geopressure zone is reached; and

halting the drilling and casing the borehole.

14. A process for drilling a borehole in a hydropressure-geopressure environment, said process comprising:

drilling the hydropressure portion of the borehole using a low weight drilling mud until the geopressure zone is penetrated;

confirming that the geopressure zone has been penetrated and setting the protective casing into the penetrated portion of the geopressure zone;

continuing to drill the borehole using an increased weight of drilling mud.

15. The process of claim 14 wherein the trend of the formation density is measured using a logging technique that responds to density and a shift in the measured density is used to confirm that the borehole has penetrated the geopressure section.

16. The process of claim 14 wherein a logging technique measuring differential pressure is used to detect geopressures.

17. The process of claim 14 wherein a logging technique that responds to penetration rates is used to detect geopressures.

18. The process of claim 14 wherein a logging technique that measures temperatures is used to detect geopressures.

19. The process of claim 14 wherein the desired depth of penetration into geopressure zone is drilled before protective casing is installed.

20. The process of claim 14 wherein a desired overbalance is maintained as geopressures are encountered and penetrated by increasing the mud weight while drilling until the desired depth into the geopressure zone is penetrated before protective casing is installed.

21. The process of claim 14 wherein a portion of the geopressure zone is drilled without protective casing and the mud weight is increased to maintain the desired overbalance until the mud weight equals the formation fracture gradient.

22. The process of claim 14 wherein the trend of the formation porosity is measured using a logging technique that responds to porosity and a shift in measured porosity is used to confirm that the borehole has penetrated geopressures.

23- The process of claim 14 wherein the mud weight is maintained between 9.5 and 11.0 p.p.g. until a kick is encountered.

24. A process for drilling a borehole in a hydropressure-salt dome environment in which sheath is encountered, said process comprising:

drilling the surface portion of the borehole and setting surface casing;

continuing to drill the borehole using a mud weight hav- 18 ing a 0.5 to 1.5 p.p.g. overbalance in the hydropressures until the first sheath kick is observed; halting the drilling and controlling the kick by increasing the weight of the mud; confirming that the kick was a result of the borehole penetrating a sheath formation; and completing the borehole. 25. The process of claim 24 wherein the borehole protective casing is installed into the sheath and the borehole is then completed.

References Cited UNITED STATES PATENTS 6/1966 Hottman 1666 OTHER REFERENCES JAMES A. LEPPINK, Primary Examiner. 

